There’s quite a lot of evidence that gases are a reasonable proxy for particles below 5um as summarised here https://t.co/JK0zudEz6H
Need to consider survival, dose-response etc, and be careful how to interpret CO2 in real spaces as uninfected also exhale it. But it’s useful.

— Prof Cath Noakes 😷 💙 (@CathNoakes) January 31, 2021